How can I transfer mail accounts to another user?

[durija]

I've several email accounts that I'm accessing through "Mail" on user "B". I'd like to transfer all of the accounts to user "A". I don't just want to set the accounts up for A. I want to actually transfer all of the old mail to A. I will then delete it all from B. None of my searches has led to an answer to this exact scenario. Looking for help.